The need for high-resolution analog-to-digital converters (A/D) has increased the use of a noise shaping technique based on oversampling and sigma-delta modulation. The resolution of a sigma-delta-A/D modulator is determined by the oversampling ratio (M) and the order of the noise shaping function. There are, however, severe stability problems associated with high-order (&gt;2) sigma-delta modulators.
The stability of high-order modulators is dependent on the amplitude of an incoming signal. When the modulator assumes an unstable mode of operation, the voltages of its integrators increase abruptly, and the bit stream from the output of the modulator begins to oscillate independently of the input signal. The high-order modulator does not return to the linear operation range even though the input would return to the stable operation range. In known solutions to the problem, the modulator is maintained stable by resetting all integrators during excessive input signal values or by accurately limiting the voltage range of the integrators within the stable operation range.
The problem with the limiting of the voltage range of the integrators is the technical realization. Above all, it is necessary to avoid any unnecessary circuits in the first stages of the modulators, in which the sensitivity of the modulator is at highest. With a signal-to-noise ratio exceeding 16 bits (98 dB), the required circuits are difficult to effect. The sensitivity of the modulator to interferences decreases rapidly in the subsequent integrator stages, and therefore it is advisable to make the additional circuits in these stages.
In addition, the resetting of all integrators in a multiple-feedback (MF) type modulator causes a reduction in the power of the bit stream from the modulator output during input signal levels above the stable operation range. As a result of this, values obtained from a decimalization filter following the modulator are smaller than within the normal operation range. This is very detrimental in practical applications. It is normally desired that the output is saturated to a maximum value when the amplitude of the input signal exceeds its operation range. In a feedforward (FF) type modulator the resetting of the integrator stages can be used without the problems described above.